We finally got this project taken care of. His approach was just as you suggested, inserting a 19" sewer pipe into the old distribution box and making it long enough to bring it close to the surface for future service. We filled the space caused by the difference in diameter with concrete.
I also built a manifold so that I could use 3 of the Simtech filters in the septic tank. He'd never seen this before, but agreed to install it for me. I can't see why it won't work.